1. Field of the Invention
The invention relates to methods and apparatus for producing thin films on substrates by vapor deposition (e.g., sputtering). The invention pertains to improving the accuracy of the deposited film thickness across the substrate (e.g., by improving the uniformity of the film thickness across the substrate where it is desired to deposit a film having uniform thickness).
2. Discussion of the Related Art
Thin film coatings are typically produced by various vapor deposition methods (such as sputtering, CVD, and electron beam evaporation) in which the substrate to be coated is passed through a vapor of the coating material and accumulates a thin film through condensation of the vapor. For many applications, such as optical films for EUV (extreme ultra violet) lithography, it is desirable that the coating be very uniform in thickness (e.g., with no more than 0.1% variation in thickness across the coated substrate). Multilayer coatings for EUV optics are commonly applied using DC magnetron sputtering.
FIG. 1 is a side cross-sectional view of a DC magnetron sputtering system, and FIG. 2 is a cross-sectional view of the FIG. 1 system taken along line 2xe2x80x942 of FIG. 1. The system of FIGS. 1 and 2, described in pending U.S. patent application Ser. No. 08/607,054, filed Feb. 22, 1996 by Vernon and Ceglio (assigned to the assignee of the present application), includes housing 10 (which has a cylindrical sidewall) and two rectangular sources (of sputtered atoms) located 180 degrees apart (relative to the system""s vertical central axis through the center of shaft 6) at opposite sides of housing 10. One source is surrounded by chimney 2; the other is surrounded by chimney 2A. Chimneys 2 and 2A limit the deposition zone for each source (in which sputtered atoms can be deposited on substrate 11 or 12) to the area directly above the target (3 or 3A) of each source. The two substrates (11 and 12) are held face down on rotatable platter 5 above the sources, at locations 90 degrees apart with respect to the axis of shaft 6.
Multilayers (alternating layers of two different materials) can be deposited on each of substrates 11 and 12 by sweeping the substrates across the sources (by controlled rotation of shaft 6 and hence platter 5 relative to the stationary housing 10 and the stationary sources).
More specifically, the system of FIGS. 1 and 2 includes a first source comprising magnetron 1, chimney 2, and target 3 positioned within chimney 2 in the electric and magnetic fields produced by element 1 (such that ions present within chimney 2, e.g., ions created within chimney 2, will accelerate toward and be incident on target 3). In response to collisions of the ions (which can be argon ions) with target 3, a vapor of sputtered atoms 4 is produced in the volume surrounded by chimney 2. Some of atoms 4 will be deposited on the downward-facing surface of substrate 11 (or 12), when the substrate (11 or 12) is exposed to sputtered atoms 4 in chimney 2.
Similarly, the system also includes a second source comprising magnetron 1A, chimney 2A, and target 3A positioned within chimney 2A in the electric and magnetic fields produced by element 1A (such that ions created within chimney 2A will accelerate toward and be incident on target 3A). In response to collisions of the ions with target 3A, a vapor of sputtered atoms 4A is produced in the volume surrounded by chimney 2A. Some of atoms 4A will be deposited on the downward-facing surface of substrate 11 (or 12), when the substrate (11 or 12) is exposed to sputtered atoms 4A in chimney 2A.
Each of two substrate holders 9 (only one of which is visible in FIG. 1) is fixedly mounted to the lower end of one of shafts 8 (only one of which is visible in FIG. 1) so as to fit in an orifice extending through platter 5. Substrate 11 is mounted on substrate holder 9, with a downward facing surface to be coated. Shaft 8 is rotatably connected to spinner 7, so that spinner 7 can cause shaft 8, holder 9, and substrate 11 to rotate as a unit relative to platter 5, whether or not platter 5 is itself rotating relative to housing 10. Similarly, substrate 12 (shown in FIG. 2 only) is mounted on a substrate holder 9 (not visible in FIGS. 1 and 2) in turn fixedly mounted to a shaft 8, and the shaft is rotatably connected to a spinner (identical to spinner 7). During operation, elements 1, 2, 3, 1A, 2A, and 3A remain stationary within housing 10, while platter 5 rotates to sweep substrates 11 and 12 sequentially across chimneys 2 and 2A (typically while substrates 11 and 12 are rotated about their centers by the spinners relative to platter 5).
To deposit typical multilayer coatings on the substrates, atoms 4 (in chimney 2) are different (i.e., have a different atomic weight) than atoms 4A in chimney 2A. In some implementations, atoms 4 are Molybdenum atoms and atoms 4A are silicon (or beryllium) atoms (and magnetrons 1 and 1A produce a plasma of ultrapure Argon ions at a pressure of about 1.00 mTorr, with source powers of 360 W and 170 W, respectively, for magnetrons 1A and 1). Platter 5 is rotated within housing 10 (at a first rotational speed) while each substrate spins (at a speed much greater than the first rotational speed) relative to platter 5. During each revolution of platter 5 relative to housing 10, each of substrates 11 and 12 sweeps sequentially across chimney 2 and chimney 2A, so that one layer of atoms 4 and then one layer of atoms 4A condenses on each substrate. The thickness of each layer is determined by the time that the substrate is exposed to the vapor (4 or 4A), which is in turn determined by the substrate transit velocity. The arrangement of substrates 11 and 12 and chimneys 2 and 2A is such that no more than one substrate is over one source at any time. Therefore, the two substrates can be independently coated with identical or completely different multilayer structures.
By rapidly spinning substrate 11 (or 12) about its own axis of symmetry relative to platter 5, better azimuthal uniformity of the condensed coating can be achieved. However, radial non-uniformities in coating thickness typically result.
To compensate for radial non-uniformities in coating thickness, a carefully shaped mask can be inserted between each substrate and the sputtered atom vapor to which the substrate is exposed. However, such a masking operation requires tedious iteration to determine the optimal shape of each mask, and can be impractical for cases in which very high uniformity is required. Use of masks also prevents independent deposition of two different coating distributions on a masked substrate as the masked substrate sweeps sequentially across two sources during a single platter rotation. Also, a mask is not suitable for a substrate whose center must be coated, since the mask cannot be perfectly positioned at the center of rotation of a spinning substrate, and thus a small spot at the center of the masked substrate will be coated with the wrong thickness.
Another approach is to use a xe2x80x9cbafflexe2x80x9d which is a shaped piece of metal that is stationary with respect to the source (unlike a mask which moves together as a unit with the substrate, as the substrate moves relative to the source) and is present between the substrate and the sputtered atom vapor to which the substrate is exposed. Use of baffles allows independent deposition of two different coating distributions on a masked substrate as the masked substrate sweeps sequentially across two sources during a single platter rotation. However, use of baffles prevents independent deposition of different coating distributions on two substrates, each of which sweeps sequentially across a source during a single platter rotation. As will be apparent from the description below of the invention, the present invention avoids use of baffles or masks, allows independent deposition of two different coating distributions on a substrate as the substrate sweeps sequentially across two sources during a single platter rotation, and also allows independent deposition of different coating distributions on two substrates, each of which sweeps sequentially across a source during a single platter rotation.
An alternative technique for improving coating thickness uniformity is described in the above-cited U.S. Patent Application by Vernon and Ceglio. In accordance with this technique, the platter velocity is modulated (as a function of time) while the substrate sweeps across each source, in such a manner to produce a desired radial variation in coating thickness on the substrate (typically an acceptably small radial variation in coating thickness on the substrate). Vernon does not teach how to determine a non-constant platter velocity (platter velocity which varies as a function of time) so as to produce a desired radial variation in coating thickness on the substrate, and apparently contemplates that this would be determined in trial and error fashion or precalculated in some unspecified fashion. However, the trial and error empirical approach to optimizing the platter velocity function (platter velocity as a function of time) in an effort to achieve a desired radial variation in coating thickness (e.g., an acceptably uniform thickness) has proved to be incapable of achieving adequate uniformity on acceptably small substrates for some applications (e.g., the 0.1%, or better, uniformity needed for EUV lithography) without use of baffles or masks.
Until the present invention, it had not been known how to achieve deposited coating thickness uniformity of better than 0.44% across typical substrates, or how to achieve coating thickness having a precisely predetermined (nonuniform) profile across typical substrates (including curved substrates such as EUV optics as well as flat substrates).
In preferred embodiments, the invention is a method for depositing a thin film with highly uniform (or highly accurate custom graded) thickness over a substrate surface, by sweeping the substrate across a region containing a vapor of the coating substance (referred to as a vapor deposition xe2x80x9csourcexe2x80x9d of coating material) with controlled velocity (where the controlled velocity is determined in accordance with the invention). The method includes the steps of:
measuring the source flux distribution (using a test piece that is held stationary in a position in which it is exposed to the source); calculating a set of predicted film thickness profiles, each film thickness profile assuming the measured source flux distribution and a different one of a set of sweep velocity modulation recipes (each xe2x80x9csweep velocity modulation recipexe2x80x9d indicating sweep velocity of a substrate relative to the source, as a function of time during a time interval in which the substrate sweeps across the source, the sweep velocity being either time-varying or constant); and then determining an optimal (or nearly optimal) sweep velocity modulation recipe to achieve a desired thickness profile (typically by selecting the velocity modulation recipe which corresponds to the predicted film thickness profile which best matches the desired thickness profile). Preferably also, a thin film having the desired thickness profile is deposited on a substrate surface by sweeping the substrate across the source with the optimal velocity modulation recipe.
An aspect of the invention is a practical method of measuring the flux distribution from the source with high accuracy. Another aspect of the invention is a method for calculating what velocity modulation recipe is needed to give the desired thickness profile on the substrate. Both aspects of the invention are applicable not only to flat substrates, but also to both concave and convex curved optics (i.e., optics having nonzero curvature).
Preferably, a computer is programmed to process measured flux distribution data (for a given source or set of sources) to generate a set of predicted coating thickness profiles, each corresponding to a different substrate sweep velocity function, and to allow the user to conveniently determine an optimal substrate sweep velocity function (for achieving a predetermined coating thickness profile on each substrate). In general, each sweep velocity function specifies how the sweep velocity varies over time. In a preferred implementation, the computer is programmed to implement an algorithm in which many sweep velocity function parameters (for example, the speed at which each substrate spins about its central axis as it is swept across the source by a rotating platter) are (or can be) varied or set to zero.
In preferred implementations, the computer is programmed to have a user interface which displays predicted peak-to-valley coating thickness error for each of a number of substrate sweep velocity functions (preferably in the form of a contour map), and a cursor. By manipulating an input device (e.g., a mouse) the user can select any of a number of points on the thickness error display, and in response to each selected point, the computer displays a graph of predicted coating thickness as a function of position on the substrate (for the corresponding sweep velocity function). By inspecting the displayed graph for each of several points on the thickness error display, the user can conveniently determine an optimal substrate sweep velocity function (for achieving a predetermined coating thickness profile on the substrate).
The inventors have recognized that slow spin speeds for the substrate (while the substrate is swept across the source) give rise to a ripple effect (rapid oscillation of coating thickness as a function of radius across the substrate), which may or may not be acceptable for specific applications.